This invention relates to electrochemical cells, and, more particularly, to lithium oxide/carbon cells used in spacecraft.
Metal oxide/carbon electrochemical cells, particularly lithium oxide/carbon cells, are useful for the storage of electrical charge in applications requiring a high storage capacity in a small volume. One form of such electrochemical cells typically has an active cell having an anode of carbon particles on a copper current collector, a cathode of lithium oxide-based particles on an aluminum current collector, a porous polymeric separator disposed between the anode and the cathode, and an electrolyte saturated into the separator. Multiple parallel layers of the anode, cathode, and separator may be made in a planar form that are rolled into a spiral and placed into a sealed metallic housing having feed-throughs for leads connected to the anode and cathode. A number of the individual electrochemical cells are connected together to form batteries of the required voltage and current characteristics.
An objective of electrochemical cell design for spacecraft applications is to reduce the weight and volume of the electrochemical cell as much as possible. One result is that as cell design advances, the heat produced during the charging/discharging cycle is concentrated into an ever-smaller volume. Heat removal from the electrochemical cell to a radiator is therefore a problem in these cases. The removal of heat from the anode, cathode, and separator of the active cell to the housing is one step of this heat removal process.
Many conventional heat-removal techniques from the active cell components used in other applications are not suitable for use in a weightless environment. Others are not suitable because they require major modifications to the internal structure of the electrochemical cell.
There is therefore a need for a better approach to the design of metal oxide/carbon and comparable electrochemical cells to achieve a faster heat removal from the active cells, thereby enabling the continuing reduction in size and closer packing of the electrochemical cells that form a battery. The present invention fulfills this need, and further provides related advantages.
The present invention provides an electrochemical cell with improved heat removal from the active cell components within the housing, and an approach for preparing and using the electrochemical cell. The approach does not require significant changes to the physical structure of the active elements of the electrochemical cell, yet accomplishes improved heat removal in an isotropic fashion. Substantially no weight is added to the electrochemical cell, and the weight may actually be reduced by employing the present invention. The electrochemical cell may be made more compact. The approach may be used for both spacecraft and terrestrial electrochemical cells.
In accordance with the invention, an electrochemical cell comprises an active element, comprising an anode comprising carbon, a cathode comprising a metal oxide, preferably a lithium-containing oxide, a separator between the anode and the cathode, and an electrolyte disposed between the anode and the cathode. There are typically multiple sets of these active elements arrayed together. The electrochemical cell further includes a sealed housing having an interior in which the active element is received, and a gas filling a gas space of the sealed housing. The gas space is a remainder of the interior of the sealed housing that is not filled by the active element. The gas comprises from about 10 to about 100, preferably more than about 25, percent by volume of a conductive gas selected from the group consisting of hydrogen, helium, and neon, and mixtures thereof. Hydrogen and/or helium are preferred.
In one embodiment, the anode comprises a copper anode current collector, and carbon particles supported on the anode current collector. The cathode comprises an aluminum current collector, and metal oxide particles supported on the anode current collector. The separator comprises a layer of microporous polypropylene. The electrolyte solution typically comprises a mixture of organic carbonates and LiPF6. The sealed housing comprises a metallic material. Leads to the anode and cathode extend through the housing walls.
The active elements may be arranged in any operable form. In a preferred form of most interest to the inventor, the anode, the cathode, and the separator are planar and are rolled into a spiral.
The use of a highly thermally conductive gas within the gas space of the sealed housing allows heat to be conducted rapidly from the active elements to the wall of the sealed housing, which thereby acts as an intermediate thermal sink for conduction to an external radiator. The conduction occurs directly from the heat-generating source to the wall in a non-mechanical fashion, rather than through an indirect path. The thermal conduction through the gaseous phase is isotropic, and achieves thermal contact to the entire surface of both the active elements and to the wall of the housing. In the spiral cell design, heat is conducted from the edges of the anode, cathode, and separator, rather than perpendicular to the flat surfaces of these elements. Mechanical weight-adding elements such as cell sleeves and thermal fins are not required by the present approach (although they may be used if desired), nor are the complex mechanical attachments associated with these mechanical heat transfer elements. The effectiveness of the gaseous conductive medium is not altered by its orientation and by the presence or absence of gravity, as would be the case for a liquid medium. The gaseous conductive medium does not change substantially over extended periods of time, as might be the case for a liquid medium. Liquid thermal conductors may puddle and form undesirable thermal blockages, which cannot happen with the gaseous conductive medium.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.